The present disclosure relates to optical fiber amplifiers.
Conventional optical fiber communications typically use dense wavelength division multiplexing (DWDM). DWDM allows a plurality of light streams having distinct and finely spaced wavelengths to propagate together, e.g., in a single-mode fiber. DWDM therefore provides an increased bandwidth for an optical fiber network.
DWDM implementations typically include the use of DWDM filters. DWDM filters can combine (e.g., multiplex) a plurality of separate light streams having finely spaced wavelengths into a single-mode fiber. DWDM filters can also separate (e.g., demultiplex) a combined light stream (e.g., a multiplexed signal) exiting, for example from a fiber, into a plurality of separate light streams each having one or more distinct, spaced wavelengths.
Conventional optical networks using DWDM typically include optical amplifiers capable of amplifying multiple light streams simultaneously. Rare earth doped fiber optical amplifiers, e.g., erbium doped fiber amplifiers (EDFA's), are commonly used in DWDM networks, although other types of optical amplifiers can also be used. When a multiplexed optical signal propagates through an EDFA, each light stream is amplified by a particular amount independently without interaction among the propagating light streams.
An erbium doped fiber (EDF) is a form of a single-mode fiber, having a core that is heavily doped with the rare earth element erbium. Conventional EDFA's include a pump laser. The pump laser provides a pump light to the erbium doped fiber to provide amplification. For example, when pump light at 980 nm or 1480 nm from a pump laser is transmitted into an EDF, erbium atoms absorb the pump light, pushing the erbium atoms into excited states. When stimulated by light streams, for example an input optical signal having particular wavelengths, (e.g., in a C-band (1528-1570 nm) or an L-band (1570-1620 nm)), the excited atoms return to a ground or lower state by stimulated emission. The stimulated emission has the same wavelength as that of the stimulating light (e.g., if the stimulating light has a wavelength of 1528 nm, the stimulated emission will also have a wavelength of 1528 nm). Therefore, the optical signal is effectively amplified as it is propagating through the EDF. Furthermore, the EDF typically amplifies all received light streams regardless of wavelength.
The power of the output amplified signal (i.e., output power, Pout) is a function of both the power of the input signal (i.e., input power, Pin) and the power of the pump light (i.e., pump power, Ppump). The output power and the input power are related by a gain G. The gain G is the ratio of the output power to the input power, or:Pout=G×Pin,  (1)where gain G is proportional (linearly or non-linearly) to the pump power Ppump.
In some implementations, a given optical amplifier can be configured to maintain a constant gain during operation. For example, an input signal of an EDFA may initially have 20 separate channels or light streams. Later, the input signal may have 18 channels because two channels are dropped before the input signal enters the EDFA (e.g., using a demultiplexing DWDM filter). If the EDFA is supplied with the same pump power, the output power per channel will increase since a smaller number of channels share the same pumped energy. Similarly, if two channels are added such that the input signal now has 22 channels (e.g., using a multiplexing DWDM filter), without changing the pump power, the output power per channel will decrease. To keep the output power per channel constant, the pump power Ppump, can be adjusted. Consequently, the total output power Pout varies as the total input power Pin varies, but gain G (e.g., as calculated using Eq. (1)) is constant.
However, in some other implementations, a given optical amplifier is configured to maintain a constant output power Pout. For example, a DWDM signal can be demultiplexed such that an input signal to an EDFA has only one channel. This single channel input signal is amplified by the EDFA, and the amplified output signal from the EDFA is fed to a detector.
The output power level should match the sensitivity and the dynamic range of the detector. If the output power is too low, it cannot be properly detected. Additionally, if the output power is too high, it can cause damage to the detector. When the input power varies, for example, as a result of channels being added or dropped, e.g., in a stage in the network prior to the current EDFA, the output power should be maintained in order to provide the same performance. An abrupt change in the input power can also be caused, for example, by network reconfigurations, failures or recovery from failures. Thus, the gain G varies as the total input power Pin varies, but the total output power Pout in Eq. (1) is constant. In order to keep the output power Pout constant, the pump power Ppump is adjusted, typically using a detected Pout.
In another example, in some DWDM systems, an input signal to an EDFA can have more than one channel. The output amplified signal from the EDFA is fed as an input signal to the next stage in the network. The total power of the input signal to the next stage needs to be at a level predetermined for that stage. Therefore, the EDFA again has to provide a constant output power Pout instead of a constant gain G.
Either a constant gain or a constant output power can be provided by properly controlling the pump power. However, an EDFA has a finite response time to the pump power change. Consequently, transient spikes can occur in the output power that can include a power overshoot or undershoot or both.